Human alveolar macrophages have cell surface receptors which are capable of binding three human leukocyte granule glycoproteins: leukocyte elastase (HLE), leukocyte chymotrypsin-like enzyme (cathepsin G, CG), and lacotoferrin (LF). At 4 degrees centigrade, the equilibrium dissociation constants for the three lignands are 4.0 10 to the minus 7th power M, 2.0 10 to the minus 7th power M, and 1.7 10 to the minus 6th power M respectively. Preliminary data suggest that all three ligands bind to the same surface receptor via a common carbohydrate recognition marker. Electron nicroscopic autoradiography of macrophages after exposure to 125I-labeled HLE revealed progressive accumulation of the radiolabel in phagolysosomes. Planned studies will: 1) further define the interrelationships in macrophage binding among HLE, CG, LF, and other leukocyte granule glycoproteins; 2) examine the metabolic fate of HLE internalized by macrophages; and 3) attempt to define the in vivo importance of macrophage binding of HLE released by neutrophils in the protection of lung connective tissue from proteolytic injury. These studies should provide insight into the importance of macrophages in lung protease defenses. The results should also clarify this potentially important type of macrophage-neutrophil interaction.